Inflatable stoppers for closing pipes, e.g. gas and water supply pipes, are known. Inflatable stoppers are balloon-like elements which are inserted into an opening in a pipe while deflated, where after the inflatable stopper is filled with pressurized medium so that it inflates to a widened state and closes the passage of the pipe. The pressurized medium may be compressible or incompressible, for example compressed air, nitrogen, hydraulic oil or water. The balloon-like element is applied radially against the inner wall of the pipe with sufficient inner-pressure to achieve an adequate closing pressure. The balloon-like element generally is provided with fibrous reinforcement to be sufficiently strong to withstand the pressure.
Useful methods of manufacturing fiber reinforced inflatable stoppers for pipes are discussed in U.S. Pat. No. 5,477,886 and WO 2011/009630.
Generally, manufacturers of inflatable stoppers need to comply with different technical regulations for safety and reliability of gas or water supply. As the requirements of the technical regulations are increasing, the manufacturers need to enhance resistance and durability of their products and adapt them according to these requirements.
Inflatable stoppers can be formed from rubber compositions containing natural and/or synthetic rubbers as raw materials. Synthetic rubbers are polymers synthesised from petroleum byproducts. Natural rubber (NR), coming from latex of Hevea brasiliensis, is mainly poly-cis-isoprene. Although it exhibits many excellent properties in terms of mechanical performance, natural rubber is often inferior to certain synthetic rubbers, especially with respect to its thermal stability and its compatibility with petroleum products. Light, heat, ozone, radiation, flexible deformation and copper, manganese and other metals can promote the aging of rubber. One drawback of natural rubber is that it is more susceptible to ozone attack than synthetic rubbers. To diminish drawbacks and enhance properties, rubbers are usually vulcanized or reinforced with e.g. carbon black as filler.
Another drawback of natural rubber is that it is sensitive to electrostatic charge. In case natural rubber is used as a base material for the inflatable body of an inflatable stopper, this charge can cause sparks in a pipeline, which could lead to an explosion hazard.
Rubber compounds, whether in continuous or intermittent use or in storage, are susceptible to ozone attack. This ozone attack on statically or dynamically employed rubber goods becomes apparent by the development of penetrating cracks which progressively become deeper with time of exposure to the atmosphere, which practically always contains ozone in trace amounts. Such cracks in inflatable stoppers may cause failure. In case new inflatable stoppers are stored for substantial periods before use, such cracks may cause failure within a relatively short time after the stoppers are put into use. Other atmospheric factors, such as sun and humidity also contribute to the deterioration of stressed rubber, but it has been found that the useful life of rubber and rubber-like articles can be greatly prolonged where the effects of ozone can be counteracted.
Ozone is generated from oxygen in the air by ultraviolet light, from sunlight, or by high voltage discharge. Concentrations in rural areas vary from 1 to 5 parts per hundred million, and in larger cities, concentrations as high or higher than 50 parts per hundred million (pphm) have been measured.
The allotropic form of oxygen (O3) has a considerably greater effect on rubber than oxygen (O2) itself. Only a few pphm ozone in air can cause rubber cracking, which may destroy the usefulness of elastomer products. Degradation results from the reaction of ozone with rubber double bonds.
Many prior art attempts have been made to combat ozone cracking in vulcanized rubber compounds and/or synthetic rubbers. Examples include GB1312636 and WO2016/130880. However, these options are generally not applicable to non-crosslinked natural latex rubber, as the rubber latex is a water dispersion before application, and non-crosslinked after application. Further, it has been suggested in the art to provide a protective surface on rubber by introducing substances into the rubber which migrate to the surface thereof to form a film thereon, but this has proven to be of little value during dynamic deformation of the rubber.
Many compounds possess the property of preserving rubber, when incorporated therein, against attack by oxygen, light and heat. Such compounds are termed antioxidants. Several additives have been described to work as antiozonants. The protective effect may result from a reaction with ozone, in which case the term used is chemical antiozonant; or in the case of physical antiozonant an effective barrier against the penetration of ozone at the rubber surface will be provided.
GB771628A relates to the bonding of a reinforcing cord to rubber. It is recognized that the presence of ozone in the air contributes to the degradation of the rubber and also of the bond between the cord and the rubber. The disclosure relates to synthetic rubbery polymer.
U.S. Pat. No. 5,439,032A relates to an elongated cylindrical stopper housing for attachment to a gas pipeline in an air tight engagement which may be accomplished through a commercially available drilling and tapping machine and by an air tight clamp assembly. This patent document does not disclose an inflatable stopper of ozone resistant natural rubber latex.
US2013/186477 relates to stoppers made from nylon, which is a material unrelated to non-vulcanized rubber.